Dynamic microchamber for measuring formaldehyde emissions

ABSTRACT

A method and apparatus for measuring the formaldehyde emission of composite wood products bonded with urea-formaldehyde adhesives employing the combination of a small sample chamber and an electrochemical sensor.

This application is a division of application Ser. No. 07/599,426, filedOct. 18, 1990, now U.S. Pat. No. 5,286,363.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus and method for measuring in ashort period of time formaldehyde emissions from composite wood productsbonded with urea-formaldehyde resin adhesives. The inventionparticularly relates to apparatus and method for monitoring formaldehydeemissions of small samples of a composite wood product that givesresults equivalent to these obtained using the large chamber test methodadopted in 24 C.F.R. 3280.208 as the industry standard for measuringformaldehyde emissions from composite wood products.

2. Description of Related Technology

Composite wood products made from urea-formaldehyde (UF) resinadhesives, such as particleboard or medium density fiberboard (MDF),find use in a variety of applications. In many of these applications,the wood products are used in areas of restricted or limitedventilation, such as in the construction of mobile homes. Sinceformaldehyde off-gas,sing or emission from UF resin bound composite woodproducts is a potential problem in such applications, standards, such asthose promulgated by the United States Housing and Urban DevelopmentAgency at 24 C.F.R. 3280.208, have been established defining thepermissible maximum level of formaldehyde emission from UF bondedcomposite wood products. Unless wood products made with a UF resinadhesive meet these emissions standards, they cannot be certified foruse and sale in the regulated applications.

A variety of methods have been developed to measure formaldehydeemissions from the planar surfaces of such composite wood products.Basically, the formaldehyde testing methods (FTM) fall into twocategories: full scale tests, which are designed to give resultscomparable to the environment encountered in actual use, and lab tests,ostensibly for quality control monitoring, designed to mimic the resultsobtained using the large scale test protocol. The first full scalemethod, the FTM-2 test, was developed by the Hardwood PlywoodManufacturers Association (HPMA) and the National ParticleboardAssociation (NPA). This test has since been modified and adopted in 24C.F.R. 3280.208 as the standard test for determining formaldehydeemissions from particleboard products such as underlayment, mobile homedecking, industrial board and the like. See, "Large Scale Test Methodfor Determining Formaldehyde Emission from Wood Products--Large ChamberMethod FTM-2--1985", National Particleboard Association et al., Nov. 11,1985 which is herein incorporated by reference. Wood products which showexcessive levels of formaldehyde emission under this test protocolcannot be certified for sale as part of the NPA and HPMA GrademarkProgram for particleboard and hardwood plywood.

The FTM-2 test uses a test chamber of at least 20 m³ (at least 800 ft³)into which a plurality of full-sized (48-60 inches in width) panels areplaced spaced apart by about 6 inches. Air is moved through the chamberat a rate sufficient to assure good mass transfer (eddy diffusion masstransfer) from the entire surfaces of the panel samples. Make-up airalso is introduced into the chamber, at a rate relative to the testchamber volume, to achieve a constant gas hourly space velocity (GHSV)or rate of renewal (Q/V) as calculated by dividing the make-up air flowrate (Q) by the test chamber volume (V). Under the regulatory programdifferent board products are tested at different product loadingconditions.

The boards first are conditioned for 7 days at 75° F. (about 24° C.) and50% relative humidity prior to testing. The samples then are placed intothe test chamber and after 16-20 hours, the level of formaldehydeemission in an FTM-2 test system is presumed to have achieved steadystate. The steady state level of formaldehyde emission is thendetermined by measuring the formaldehyde concentration (C_(S)) in thecirculating air. Formaldehyde concentration in the chamber air ismeasured in the FTM-2 test by bubbling a known volume of the chamber airthrough an impinger containing an aqueous solution of 1% sodiumbisulfite. The bisulfite solution is then analyzed using thechromotropic acid method and a spectrophotometer to determine itsformaldehyde concentration. Using known techniques, the measured valueis converted into a formaldehyde emission value for the tested beardsand compared against the established standards.

Although the FTM-2 test provides an accurate indication of theformaldehyde emission characteristics of the tested panels under thetest conditions, the FTM-2 method is not suitable for routine processquality monitoring in a manufacturing facility. Not only is the longtest interval prohibitive for measuring C_(S), including the week-longconditioning period and almost a full day for the test, but the FTM-2test is also deficient as a method for assessing equilibriumformaldehyde emission (C_(eq)) and the mass transfer coefficient (K) forthe conditioning period and almost a full day for the test, but theFTM-2 test is also deficient as a method for assessing equilibriumformaldehyde emission (C_(eq)) and the mass transfer coefficient (K) forthe composite wood products.

A laboratory test purportedly developed to provide data that correlatesto the full-sized FTM-2 test has been named the FTM-1 test. The FTM-1test (also known as "the desiccator test") is a faster test method,which after a 24 hour conditioning period, requires only about 2 hoursbefore a result is obtained. See, "Small Scale Test Method forDetermining Formaldehyde Emissions from Wood Products, Two HourDessicator Test FTM-1--1983", National Particleboard Association et al.,Oct. 10, 1983 which is herein incorporated by reference.

To conduct the FTM-1 test, a plurality of small samples are placed in adesiccator containing a petri dish with 25 ml of distilled water. After2 hours a sample of the water (about 4 ml) is analyzed by thechromotropic acid method wherein the concentration of formaldehyde ismeasured by monitoring the color of the solution using aspectrophotometer.

The FTM-1 test, however, has a number of shortcomings. First, the testprocedure relies on molecular diffusion from the test specimens into thesolution rather than on the more efficient eddy mass transfer obtainedunder conditions prevailing in the large scale test chamber. Secondly,the FTM-1 measuring method places an aqueous solution in the sameenvironment as a formaldehyde-emitting surface. It is well known thatformaldehyde emission from a composite wood product is stronglyinfluenced by the conditions of exposure, particularly the prevailinghumidity. The aqueous solution necessarily affects the humidity in thedesiccator and thus the rate of formaldehyde emission from the testedsamples.

Finally, the nature of the test is such that the same test results areobtained even when the total surface area of the wood composite samplesis changed, indicating that the desiccator does not respond in a waysimilar to the large scale test chamber. This results in part becausethe amount of formaldehyde that is absorbed by the test solution in theFTM-1 test disturbs the emission equilibrium of the composite woodproduct sample and thus affects the amount of formaldehyde emitted fromthe samples contained in the test chamber. In other words, the aqueoussolution acts as a formaldehyde sink increasing formaldehyde emissionfrom the test samples rather than simply serving as an impartial monitorof equilibrium formaldehyde concentration in the test chamber. SeeKinetics for Desiccator Jar and Alike Tests for Formaldehyde Releasefrom Particleboard, Rybicky, Jaroslav, Wood & Fiber Science, Vol. 17,No. 1, January 1985.

Because of these and other inadequacies in the FTM-1 procedure, attemptsto correlate its emission tests results with results obtained using thelarge scale chamber (FTM-2), which as noted above serves as theregulatory standard, have been less than satisfactory. Consequently,saddled with such testing inadequacies, mills producing composite woodproducts have been forced to operate with a wide production margin toensure that certified board products meeting the formaldehyde emissionlimits of the appropriate standards are consistently obtained. Moreover,because only a single measure of formaldehyde can be determined, theFTM-1 test is deficient as a quality control tool in the manufacturingenvironment even if consistent results are obtained. As noted above, atest procedure that also determine the mass transfer characteristics ofthe composite wood product is needed if it is to be useful for qualitycontrol. Therefore, it would be desirable to have an alternative test tothe large scale FTM-2 procedure that would measure formaldehydeemissions quickly (e.g., in less than about 30 to 60 minutes),conveniently and inexpensively with good correlation to FTM-2 testresults.

One might think that it would be an easy matter merely to scale down theFTM-2 test chamber while continuing to use the corresponding make-up airflow rate (GHSV or Q/V) and loading ratio, i.e., the ratio of samplearea (A) to chamber volume (V) used in the FTM-2 protocol. In otherwords, using a smaller chamber designed for operation at the sameconstant Q/V with the same corresponding A/V to imitate the large scalechamber. Unfortunately, if the air sampling and monitoring protocolremains the same as in the FTM-2 test, i.e., a sampling flow rate of 1liter/min. to ensure identical results, the minimum chamber size for theregulatory required Q/V of 0.5 would be 0.12 m³ (about 4 ft³), a notinconsequential size. The device would be smaller than the large scaletest chamber but not yet useful as a quality control monitoring methodbecause steady state conditions would still require 16-20 hours toachieve. Moreover, the reduced absolute air flow and emittedformaldehyde volume would have to be measured by some method. Throughthe impinger technique of the FTM-2 protocol could be used to measuresteady state (C_(S)) formaldehyde emission from such a scaled downchamber, it can not be used to measure an equilibrium emission levelbecause of formaldehyde mass balance and humidity problems alsoencountered with the FTM-1 method. Furthermore, the impinger techniquefor determining the steady state formaldehyde C_(S) concentration itselfrequires a minimum of about 30 minutes to conduct to ensure an accuratedetermination of the formaldehyde emission level, and this is inaddition to the time needed to reach steady state conditions.

It would be desirable to have a system for accurately measuringformaldehyde emission from composite wood products under both steadystate and equilibrium conditions in less than about 30 to 60 minutes sothat it could be correlated to the FTM-2 results and also serve as auseful quality control tool in the manufacturing environment.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an apparatus, and a methodfor measuring steady-state formaldehyde emissions from composite woodproducts in a short time period of less than about 30 minutes.

It is another object of the invention to provide an apparatus, and amethod for measuring equilibrium formaldehyde emissions from compositewood products in a short time period of less than about 60 minutes.

It is yet another object of the invention to provide a device useful formeasuring for formaldehyde emissions from composite wood products havinga test chamber with a volume of less than about 0.5 m³ and preferablyless than about 0.1 m³.

In accordance with the objects noted above and others as will berecognized by those skilled in the art, the invention relates to anapparatus and a method for measuring formaldehyde emissions fromcomposite wood products. The apparatus aspect of the present inventionconcerns an apparatus for measuring formaldehyde emissions fromcomposite wood products bonded with a urea-formaldehyde resin adhesive,said apparatus comprising in combination:

a sample chamber having a volume of lass than about 0.5 m³ for holdingat least one sample board of said composite wood product and permittingsaid board to emit formaldehyde in said chamber;

blower means for circulating air through said sample chamber and acrosssaid at least one sample board in said sample chamber; and

an electrochemical formaldehyde sensor in fluid communication with thecirculating air for rapidly measuring the concentration of formaldehydein said air without consuming any significant amount of the emittedformaldehyde.

The measuring method using the apparatus described above comprises:

flowing air for less than about 30 minutes over at least one boardsample of said composite wood product contained within a sample chamberto form a formaldehyde-containing air stream; and,

passing a portion of said formaldehyde-containing air stream over anelectrochemical formaldehyde sensor to measure the concentration offormaldehyde in said air stream, wherein said sensor does not consumeany measurable amount of formaldehyde in said air stream.

The invention provides a practical means for monitoring formaldehydeemissions within a time frame required for effective manufacturingquality control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate the construction of an electrochemical sensorthat is useful in the present invention.

FIGS. 3 and 4 show side and top views respectively of a sample chamberuseful in the invention.

FIG. 5 is a block diagram of a sampling system.

FIG. 6 is a graph showing the formaldehyde emission characteristic of ahypothetical sample of a composite wood product.

DETAILED DESCRIPTION

The present invention relates to apparatus and method for measuringformaldehyde emissions from composite wood beard products that arebended together with a high formaldehyde emitting adhesive, such as aurea-formaldehyde resin adhesive. Examples of such composite wood beardproducts include, but are not limited to, hardwood plywood,particleboard, and medium density fiberboard (MDF).

The present invention capitalizes, in part, on the recognition hat it isnot necessary to maintain the same gas loading (Q/V) and the same sampleloading (A/V) in a small sized chamber as that used in the large scaletest chamber (FTM-2) in order to obtain statistically identical testresults to the large scale chamber. Rather, as long as the ratio of themake-up air flow rate (Q) to the sample surface area (A) is heldconstant at the same value used in the large scale protocol and thechamber design accounts for adequate mass transfer, comparable resultscan be obtained. A recent article discussing calculations that supportsuch conclusions is Christensen et al., "Measuring FormaldehydeEmissions Using a Small Scale Chamber", Proceedings 23rd InternationalParticleboard/Composite Materials Symposium, Apr. 4-6, 1989 which isherein incorporated by reference. The present Invention also is premisedon the identification of a formaldehyde sensor that not only has arelatively rapid response time, but also consumes an insignificantamount of formaldehyde when performing the analysis.

Thus, the present invention provides a combination of a rapid responseelectrochemical formaldehyde sensor and an integrally connected, andspecially designed small sample chamber. A key aspect of the presentinvention is that the apparatus can be used both to measure steady stateformaldehyde emission values (C_(S)) for a composite board sample at adesired gas loading (Q/V) as well as to determine an equilibriumemission value (C_(eq)) for the sample, i.e., the formaldehyde emissionin a closed system with no make-up air flow (Q/V=0). Most importantly,both of these determinations can be made in a very short time frame, onthe order of minutes instead of hours as required by the large scaletest chamber, and the measured values have a high statisticalcorrelation to results which would be obtained on the same board samplesusing the testing protocol required by FTM-2.

An electrochemical formaldehyde sensor with a rapid response time,operating on the voltametric principle is one key element of the presentinvention. Such sensors are relatively inexpensive and, mostimportantly, can determine the formaldehyde content of a gas sample withsuch a sufficiently low (essentially undetectable) level of formaldehyderemoval from the gas sample as to leave the formaldehyde concentrationof the gas essentially undisturbed. Unlike prior formaldehyde measuringmethods used in determining formaldehyde emission of composite woodproducts, which methods, if not done on a large scale, affect the gasphase equilibrium due to high gas sampling requirements, theelectrochemical formaldehyde sensor used in the present invention, byneeding only a very small (essentially undetectable) quantity offormaldehyde to assess gas phase concentration, effectively eliminatesthe requirement of a minimum test chamber size. The sensor also does notadd any significant amount of water into the test chamber and thusavoids creating a measurement error due to a varying humidity conditionin the test chamber. Consequently, such measurement technique makespossible a significant reduction in the size of a test chamber formeasuring formaldehyde emissions of composite wood board products; yetretains the ability to obtain results which are statistically equivalentto these obtain using FTM-2.

In general, electrochemical sensors operate by passing gas moleculesfrom a sample through a diffusion medium and adsorbing the gas moleculeson an electrocatalytic sensing electrode maintained at a sensingpotential appropriate for the electrode. On the electrode, the adsorbedgas molecules react and generate an electric current in proportion totheir gas phase concentration. The sensed current can then be comparedagainst a calibration curve and used to determine the subject gasconcentration in the sample or can be calibrated using a known standardand visually displayed on a suitable meter.

An electrochemical sensing cell is a device which generates anelectrical current preferably only in the presence of the targetedcompound, e.g., the pollutant being measured. The magnitude of thiscurrent is proportional to the pollutant concentration, which may beindicated by a meter connected to the output of an amplifier whichamplifies the current from the sensing cell. An electrochemical sensingcell incorporates two electrodes, one called a sensing electrode and theother called a counterelectrode, immersed in an electrolyte. When thegas containing the targeted pollutant contacts the sensing electrode,reactions occur which cause a current to flow in a circuit comprisingthe counterelectrode, the electrolyte, the sensing electrode and anexternal lead connecting the sensing electrode back to thecounterelectrode. The magnitude of this current is proportional to thepollutant concentration in the gas. By appropriate selection ofcounterelectrode and electrolyte materials, the sensing cell may be madeselective to a particular gas species.

Depending on the species to be detected, either oxidation or reductionoccurs at the sensing electrode, and the complementary reaction occursat the counterelectrode. For example, to detect formaldehyde (CH₂ O),oxidation occurs at the sensing electrode, which preferably comprises anoble metal such as gold. Electrochemical reduction occurs at thecounterelectrode, which may comprise lead in an electrolyte of aqueouspotassium hydroxide.

A preferred sensor construction uses an external voltage bias tomaintain a constant potential on the sensing electrode relative to anonpolarizable reference counterelectrode. The term "non-polarizable"refers to a counterelectrode that can sustain a current flow withoutsuffering a change in potential. Such nonpolarizable counter-electrodesavoid the need for a third electrode and a feedback circuit. Because theoxidation and reduction potential for formaldehyde is known or readilydeterminable by the exercise of no more than routine experimentation,the bias may be set to ensure that substantially only formaldehyde isreacted at the sensing electrode.

A particularly preferred formaldehyde sensor is commercially availablefrom Interscan Corporation of Chatsworth, California under thedesignation Interscan Model LD-16 and is described in U.S. Pat. No.4,017,373. The general simplicity of the electrochemical sensors makethem significantly less expensive than other direct measurement systemssuch as IR analyzers. The description of the Interscan patent is hereinincorporated by reference but may be briefly described with reference toFIGS. 1 and 2 (from U.S. Pat. No. 4,017,373).

Electrochemical sensing cell 10 detects and measures the concentrationof formaldehyde in a selective sample of circulating air from a samplechamber containing composite beard samples. The withdrawn air samplecontaining emitted formaldehyde flows into the sensing cell 10 viaconduit 11 and exits from the cell via conduit An appropriate pump (notshown) of either the positive pressure or suction type is used to forcethe contained air gas through sensing cell 10. If formaldehyde ispresent, a current will be generated between the sensing electrodeterminal 13 and counterelectrode terminal 14. This current can beamplified and/or combined with other information concerning the sampledbeards to drive a meter or other form of display means which indicatesthe formaldehyde concentration, for example, in parts per million.

Sensing cell 10 includes a cylindrical container 15, closed at thebottom 15a, that holds immobilized electrolyte 16 and a counterelectrode 17 immersed in the electrolyte. Metal clamp 18 surrounds thecontainer 15 and serves the double function of mounting the sensing cell10 to an L-bracket 19 and of providing electrical connection to thecounterelectrode terminal 14. That terminal may comprise a thin strip offoil mounted on the outside of cylinder 15.

Wire 20 connected to the counterelectrode 17 extends through a hole inthe cylinder 15 and has an end portion 20a that is bent back underneaththe terminal strip 14. Clamp 18 covers the strip 14 and insures goodelectrical contact between the wire 20, the strip 14, and the clamp 18itself.

In FIG. 2, sensing electrode 23 is illustrated as planar. Sensingelectrode 23 is clamped between a cover 24 that seats atop the open end15b of the container 15 and manifold cap 25 to which the inlet andoutlet conduits 11, 12 are connected. Cover 24 has a central opening 26through which the electrolyte 16 can reach the sensing electrode 23. Thelower surface 25a of the cap 25 includes a recess through which the gasto be analyzed reaches the sensing electrode 23. Voltametric sensingthus is facilitated, since the sensing electrode 23 is in contact withbeth cell electrolyte 16 via the opening 26 and the gas species suppliedvia the recess.

Screen 35 supports one or more discs 36 of filter material whichfunction to ensure intimate contact between electrolyte 16 and thesensing electrode 23 To this end, screen 35 is formed of a material thatis non-reactive with electrolyte 16 and which is sufficiently rigid tosupport the filter disc 36 without becoming concave at its center.Polyester is an appropriate material for screen 35.

Disc 36 has a diameter slightly less than the opening 26a so as to fitwithin this opening. Typically disc 36 comprises a glass filter papersuch as that sold commercially. More than one such disc 36 may berequired to fill completely the space between the screen 85 and thesensing electrode 28. The electrolyte flows through the screen 35 andcompletely wets the disc or discs 86. Since these are slightlycompressed between the screen 86 and the sensing electrode 28, intimatecontact is obtained between the electrolyte that saturates the disc ordiscs 86 and the sensing electrode 28.

To prevent sloshing of electrolyte 16 within the cell 10, container 15may be filled with an inert, absorbent material 88, such as glass wool,to immobilize the electrolyte. The absence of free electrolyteeliminates undesirable sensor noise and is particularly advantageouswhen high amplifier gain is required for low concentration readings.

Only a small portion of the gas circulating through the sample chamberis needed and should be supplied to the sensing electrode 23. To thisend, a through passageway 48 is provided in the cap 25 between the inletconduit 11 and the outlet conduit 12. A pair of lateral ports 49, 50branch off from the passageway 48 and extend to the recess in cap 25mentioned above. Ports 49 and 50 are spaced apart so as to be adjacentthe edges across the recess. With such placement, some of the gasentrant through the conduit 11 will flow through the branch port 49,into the recess and then out through the port 50 and the outlet conduit12. Intimate contact between this sample gas and the counterelectrode 23thus is accomplished within the recess.

Due to the very low (essentially undetectable) formaldehyde lossesencountered with the operation of the electrochemical sensor used in thepresent invention, the sample chamber can be substantially smaller thanprevious devices yet still can be used to determine equilibriumformaldehyde emission without introducing any sampling error into thedetermination of C_(eq). A smaller sample chamber in combination with alarge emitting surface area (i.e., a high sample loading area by usingmultiple samples) can come to equilibrium quickly (e.g. in less thanabout 30 to 60 minutes) and also provide a practical means formonitoring steady state formaldehyde emission rates.

The high loading also increases the obtention rate of steady stateconditions, so that the data needed to assess fully the mass transfercharacteristics of a board sample can be gathered in a very short timeperiod. The desired sample chamber size, in accordance with the presentinvention for most quality monitoring methods is leas than about 0.5 m³,and preferably is less than about 0.1 m³. In order to obtainformaldehyde emission results which are representative of a full sized(e.g. 4"×8") sheet of the composite wood product, it is preferred thatthe sample chamber have a volume of at least about 0.02 m³ and beconfigured to hold at least three board samples, from each sheet.

A particularly useful sample chamber has a volume of about 0.044 m³.With such small chamber sizes, it is convenient to use board sampleshaving a planar surface area of about 0.45 to about 0.65 m² although thespecific board sample size chosen will depend on the actual chamber sizeused and the material being tested. With boards expected to haverelatively low mass transfer coefficients, one could use slightly largerboard samples than those used with boards having relatively high masstransfer coefficients to keep the time periods similarly short needed toreach steady state and equilibrium conditions. Because sample chamber300 places the boards in a serpentine path, the chamber can easilyaccommodate a variety of sample sizes, merely by changing the length ofthe samples, although the same sample size should be used for any giventest.

A side view of a particularly useful sample chamber is illustrated inFIG. 3 with a top view in FIG. 4 showing the placement of sample boardsin the chamber. Similar structural features boar the same referencenumber in the various figures for convenience.

Sample chamber 300 is essentially a rectangular box having top wall 301,bottom wall 302, front wall 303, rear wall 304, and side walls 305 and806. Hinged door 807 is located on front wall 809 and air blower 908with recycle conduit 910 is located on rear wall 904. Suitable insidedimensions for chamber 300 are a width of about 13.8 inches (35 era) alength of about 23.875 inches (60.6 cm) and height of about 8 inches(20.3 cm).

In combination with the arrangement of samples in chamber 300, blower308 should be of a sufficient size to circulate the air within samplechamber 300 with sufficient velocity to ensure that eddy diffusionacross the board sample surfaces is the principal mass transfermechanism and to insure the absence of formaldehyde gradients withinchamber 300. Typically, a blower capable of recirculating air at a flowrate of about 40 to 50 scfm should be sufficient for the small chamberof the present invention.

Hinged door 307 is sealed to prevent gas leakage into or out of chamber300 with any sealing means that is capable of maintaining an air tightseal around the perimeter of door 307. The illustrated sealing means isa closed cell foam 311 of the type commonly used for weather strippingaround doors and windows and a pair of wing nuts. Other door sealingmeans including magnetic seals, hemispherical silicone strips,tongue-and-groove door construction details, and virtually any materialuseful for weather stripping are also suitable.

For sampling purposes, for inflow of make-up air, for exhaust and forbypass, sample chamber 300 includes a variety of ports 312 in rear wall304 or side wall 306. Ports 312 are preferably positioned where airinput or exhaust will not disrupt the air flow over the sample boards.The ports are designed so that they can be selectively used inconjunction with the operation of chamber 300.

In FIG. 4, chamber 300 holds three board samples vertically and theports are located above the recycle conduit 310. The number of boardsgenerally used is, to a certain extent, a matter of choice. Obviously,the chamber must contain at least one board. However, in order toprovide the desired range of sample loadings in a conveniently sizedsample chamber, applicants have found that the use of three boardsamples is suitable. Three boards reach steady state and equilibriumconditions much faster than a single board. Moreover, the use of threesample boards facilitates the realization of adequate mass transferconditions in sample chamber 300. The boards are arranged so that aircirculating over the boards follows a serpentine path between the outerface of the first board and side wall 305, between the first board andthe second board, between the second board and the third board, andfinally between the outer face of the third board and side wall 306. Thesampling ports are located on rear wall 304 above and below opening 314for recycle conduit 310 and in the path between the outer face of thethird board and right wall 306. Preferably the boards are evenly spacedin chamber 300 to provide a uniform flow condition in the chamber.

The edges of a board sample of a composite wood product, andparticularly the edges of particleboard and MDF, generally have a muchhigher formaldehyde diffusion (emission) rate than the planar surfacesof the board. The edges also constitute a proportionately greaterfraction of the total surface area of the board sample in the smallersized samples used in connection with the present invention than in thefull sized composite wood panels from which they come. Consequently, toobtain an accurate measurement of the formaldehyde emission of the boardfrom which the samples were obtained, the edges of the samples should besealed to prevent or retard formaldehyde emission during testing toavoid bias. Suitable sealing materials preferably include nonporoustapes and possibly non-volatile liquid sealants. A metal tape, such asaluminum tape, should be sufficient.

The boards should be positioned in chamber 300 in such a way as toprevent gas leaking and short circuiting directly from inlet to outlet.This is accomplished, for example, by sealing the boards against the topand bottom walls of chamber 300 to force the recirculating air to followthe desired serpentine flow path through chamber 300. The top and bottomedges of the sample boards may be considered as sealed if they arewedged between top 301 and bottom 302. Wedged fits are an efficient formof friction fit that unfortunately places unnecessary stress on samplechamber 300 and requires sample pieces that are accurately cut. Analternative friction fit can be accomplished more easily using a bentplate 313 disposed loosely inside chamber 300 over bottom wall 302.Preferably bent plate 313 is made of a material that does not absorb orreact to formaldehyde and is able to withstand repeated flexure withoutpermanent deformation. Particularly preferred materials include metalssuch as stainless spring steel.

FIG. 5 is a block diagram of the air flow arrangement for chamber 300including the sample flow of a sensing system according to theinvention. When measuring steady-state formaldehyde emissions (C_(S)), amake-up air pump 500 supplies ambient make-up air through an airregulator 501 and an activated carbon filter 502 via feed conduit 514into chamber 300. The carbon filter is designed to remove contaminants,particularly ambient formaldehyde, that might interfere with propermeasurement of formaldehyde emission levels in chamber 300. Flow meter503 displays and allows control over the rate of make-up air introducedinto sample chamber 300 through feed conduit 514. Any flow metercontroller should be satisfactory for use in the present invention whichpermits accurate adjustment of the make-up air flow between 0 and thatlevel needed to yield the appropriate ratio of make-up air flow tosample area (Q/A) equivalent to the regulatory testing protocol ofFTM-2, i.e. 1.9 m³ /m² -hr for MDF, 1.17 m³ /m² -hr for particleboardand 0.53 m³ /m² -hr for hardwood plywood. A suitable flowmeter isavailable from Cole-Parmer Instrument Co., Chicago, Ill. as model numberN-03227-30. Make-up air is not used when measuring equilibriumformaldehyde emission values.

Blower 512 recirculates formaldehyde-containing air in sample chamber300 via recycle conduit 504 so as to ensure adequate mixing withinsample chamber 300. Sampling line or conduit 505 is used to remove aportion of the circulating air selectively during a sampling phasethrough timer and 2-way valve assembly 506 to the electrochemical sensor508 containing an integral sample pump. Gas delivered to sensor 508 canbe discharged through exhaust 511. Alternately, ambient air is flowedinto the electrochemical sensor 508 through zero reference filter 507and the 2-way valve 506 and is discharged through exhaust 511.

During operation of the sensor unit, gas, either ambient air or chamberair is constantly flowed through the formaldehyde sensor. Proper zeroingof the sensor output is done when filtered air is flowing through thesenor. By-pass line 509, which is selectively connected to the exhaustline 511, such as by a valve, is used to route the circulated gas sampledirectly from electrochemical sensor 508 back to chamber 300. Thisarrangement would be used when measuring an equilibrium formaldehydeemission value.

Gas is exhausted from chamber 300 through valved exhaust port 510. Thedischarge ends of exhaust port 510, and exhaust port 511 of theformaldehyde sensor, are desirably positioned a sufficient distance frommake-up air pumps so as not to introduce excessive ambient formaldehydethrough filters 502 and 507. Preferably, the exhaust lines dischargeinto a room separate from the location of chamber 300 or outside.

A particularly useful feature of the present invention is that theabove-described assembly can be operated in two modes. In a first mode,steady state formaldehyde emission values for a sample of a compositewood product can be determined at any desired make-up air loading. Inits second mode, an equilibrium emission value for the sample beards canbe measured. In both modes of operation, the samples are placed inchamber 300 and blower 308 is activated. When measuring a steady stateemission, make-up air pump 500 also is activated and the flow meter isadjusted to provide the proper flow rate of make-up air through feedconduit 514 into chamber 300 to give the Q/A ratio appropriate for thecomposite wood product being tested. A portion of the recirculating airis exhausted through exhaust conduit 510 so as to maintain a proper massbalance. When a formaldehyde measurement of the recirculating air istaken by flow of a portion of the recirculating air through conduit 505,it is subsequently exhausted through exhaust conduit 511. Thus, in thesteady state mode, recirculating air is discharged through port 510 andair flowed to sensor 508 is discharged through port 511. When anequilibrium emission in being measured, exhaust ports 510 and 511 areclosed, make-up air flow into chamber 300 through feed conduit 514 isterminated and air flowed to sensor 508 through conduit 505 is returnedto the sample chamber through by-pass conduit 509. The system thusbecomes close-ended.

Instead of having three separate valve-controlled conduits 509, 510 and511, the system can be operated manually with only exhaust conduits 510and 511, which in the equilibrium mode, are placed in flow communicationto establish return line 509.

It has been observed that the electrochemical sensor used in the presentinvention has such reproducible response characteristics that it is notnecessary, when measuring the formaldehyde emission of a board sample,to wait until the output of the sensor stabilizes. Rather, the sensorcan be calibrated using a sample with a known formaldehyde concentrationby tuning the output to the known value at any time after the initialresponse reaches about 80 to 90% of the known final output. This hasbeen confirmed by calibrating the sensor to a known sample at variousresponse times from 2 minutes to 20 minutes with no statisticaldifference in the subsequent results obtained. This is an importantaspect of the method invention of the present application. Obviously, tominimize the test period, a shorter time for calibrating the sensorshould be selected. In practice, calibration and testing at a 5 minuteinterval after the initial exposure has proved suitable in the preferreddesign.

Operation of the device for measuring formaldehyde emission starts witha calibration of the electrochemical sensor such as against a samplewhose emission characteristics previously have been determined such asby using the large scale chamber or alternatively by using the smalltest chamber in combination with a conventional liquid absorption, i.e.,impinger, test. In a preferred calibration technique, the known samplesare inserted into chamber 300, blower 310 is activated and the make-upair flow rate appropriate for the composite wood product being analyzedis begun. During a time period sufficient for the known samples to reachsteady state conditions, i.e. less than about 30 minutes and typicallyabout 5 minutes, two-way valve 506 directs air from zero referencefilter 507 into sensor 508.

Once a sufficient time has elapsed for the board samples having theknown emission characteristics to reach steady state emission, thesensor is exposed to the formaldehyde source of the known concentration.This is done by setting the timer for the two-way valve for theappropriate exposure period, generally between 2 and 20 minutes withfive minutes preferably being used. A portion of the gas from chamber300 now flows through conduit 505 and into sensor 508. The flow rate ofsampled gas should be adjusted to between about 0.3 to 0.7 liters perminute, typically about 0.5 liters per minute. If the flow rate to thesensor is too low, then the sensor response time is adversely affectedand an accurate reading can not be obtained in the desired minimum timeframe. Applicants have observed that if the flow rate is too high thenthe lifetime of the sensor is adversely affected. Also, too high a flowrate is undesired when measuring the equilibrium formaldehyde emissionof a board sample. Under proper flow conditions, a sensor can beexpected to last for about 300-600 hours of testing.

The instrumentally displayed sensor output then is tuned to the knownformaldehyde emission value of the sample through span adjustment at thedesired time after exposure of the sensor to the known formaldehydesource, typically about five minutes. All subsequent determinations ofthe steady state or equilibrium formaldehyde emission, i.e.,concentration, of unknown board samples is then conducted at the sametime interval after exposure as used for the initial calibration, e.g.,about 5 minutes after exposure. Such calibrations are well within theexisting skill for one in this art and are outlined in the protocol forthe FTM-1 and FTM-2 tests.

In an alternative calibration technique, unknown board samples can beused first, the above-described procedure is repeated three times to getan average emission value for the unknown sample at the existingsetting. Then, the protocol is repeated again but instead of determiningformaldehyde concentration using the electrochemical sensor, the exhaustfrom the small chamber is routed through allow meter and into a liquidimpinger for measuring formaldehyde by a wet chemistry technique. Thepreviously measured values facilitate the wet chemistry analysis.According to the FTM-2 procedure, typically a thirty minute period forabsorbing formaldehyde in the liquid impinger should be suitable. Thesamples then are rerun as above using the electrochemical sensor. Theemission value obtained from the wet chemistry procedure then is used asthe standard for adjusting the span on the sensor output upon rerunningthe board samples a fourth time.

Once calibrated, samples of unknown emission characteristics are loadedin the sample chamber and the air flow rates for the makeup air (ifused) and sampling pump are adjusted as desired. Any number of sampleboards can be used, but it is preferred to use a manageable number suchas 3. An odd number of sample boards is preferred so that a serpentineflow around the boards can be preserved. For a sample size of 3 boardshaving the dimensions 7.875 inches×15 inches, a sampling pump flow rateof 0.5 liters/min. in a sampling chamber of about 0.044 m³ (nominal IDof 24"×8"×14"), is suitable. Make-up air can be supplied at rates of upto about 15 liters/min. depending on the product being tested.

Prior to testing, board samples can be stored in hermetically sealedbags to arrest formaldehyde emission, and just prior to testing thesamples are preferably conditioned for about an hour. The boards areconditioned under fixed conditions of temperature and ventilation. Aconditioning temperature of 77° F. has proved suitable for producingresults comparable with results obtained with FTM-2. The testingconditions are desirably about 75°-79° F. and 46-54% relative humidity.The preferred testing conditions are 77° F. and 50% relative humidity.

Once the samples have reached steady state in test chamber, whichrequires less than about 30 minutes, e.g., the samples have been in thesample chamber with the appropriate flow rates for about 5-10 minutes,with air flow through the zero reference filter into the sensor,thereafter the time on the 1-way solenoid valve should be set to allowgas flow from chamber 300 and into the electrochemical sensor for about5 minutes before returning to air flow through the zero referencefilter. Readings are taken at intervals of 2, 4, and 5 minutes. At theend of the 5 minute-interval, the 2-way valve will automatically returngas flow through the zero reference filter. The recorded emission valueis the reading taken at 5 minutes. The reading sequence preferablyshould be repeated twice with different samples from the same board toensure accuracy of the measurement. The formaldehyde reading obtainedshould be corrected for 77° F. and 50% relative humidity. Suchcorrections are within the existing skill level for one in this art fromthe above mentioned protocol for the FTM-1 and FTM-2 methods.

As an example, if the reading at the end of 15 minutes is 0.25 ppmformaldehyde with a final temperature of 75° F. and a relative humidityof 54%, adjusting the reading for 77° F. according to the well knownBerge et al. formula would add 0.0:1 ppm for the low temperature butsubtract 0.02 ppm for the increased humidity. The corrected readingwould be 0.26 ppm formaldehyde.

It is expected that the total testing assembly and method can beautomated and through appropriate hardware and software integratedoperated with computer assistance. The basic operations, however, remainas described above.

The readings from the dynamic microchamber of the invention showexcellent correlation to readings from the FTM-2 large scale tests. Thefollowing is a list of actual readings obtained from both the FTM-2 test(which has an accuracy of 0.02 ppm) and the dynamic microchamber of thepresent invention.

Example 1--Particleboard

    ______________________________________                                        Dynamic        FTM-2                                                          Microchamber   Large Chamber                                                  (ppm)          (ppm)                                                          ______________________________________                                        0.135          0.13                                                           0.31           0.32                                                           0.12           0.13                                                           0.21           0.21                                                           0.17           0.18                                                           0.184          0.18                                                           0.138          0.14                                                           ______________________________________                                         Correlation coefficient = 0.9907                                         

Example 2--Medium Density Fiberboard

    ______________________________________                                        Dynamic        FTM-2                                                          Microchamber   Large Chamber                                                  (ppm)          (ppm)                                                          ______________________________________                                        0.462          0.46                                                           0.265          0.26                                                           0.37           0.33                                                           0.322          0.325                                                          0.215          0.185                                                          ______________________________________                                         Correlation coefficient = 0.9668                                         

As noted previously a particularly important aspect of the presentinvention is that the apparatus and method can be used for measuring, ina very short time frame both steady state (C_(S)) and equilibrium(C_(eq)) formaldehyde concentrations of a composite wood product.Consequently, the invention serves as a meaningful quality control toolfor a manufacturing facility.

Formaldehyde emissions levels of composite wood products can beinfluenced by both changes in the UF adhesive chemistry and by processconditions during manufacturing, such as the relative application levelof the adhesive and press conditions. Moreover, the emissioncharacteristic of a given board can be considered as being based on twofactors (1) the equilibrium emission characteristic of the board and theboard's mass transfer coefficient (K). This is illustrated in connectionwith FIG. 6 which plots the reciprocal of formaldehyde concentrationagainst gas loading (Q/A) for a hypothetical board sample. In the FIG.6-type plot, the intersection of the line with the y-axis at Q/A=0 isthe reciprocal of the equilibrium emission concentration (1/C_(eq)),while the slope of the line times the "y" intercept is the mass transfercoefficient (K). Unless, one has available the information shown by FIG.6 one has no way of knowing whether the level of emission is due to ahigh equilibrium emission level, generally a UF adhesive chemistryissue; or is due to a high mass transfer coefficient, generally amanufacturing issue. The present invention permits one to obtain theinformation needed to make this assessment in a very short time period.Thus, the invention constitutes a practical method for obtaining suchdata as an integral part of a mill's formaldehyde emission qualitycontrol program.

While certain specific embodiments of the present invention have beendescribed with particularly herein, it will be recognized that variousmodifications thereof will occur to those skilled in the art and it isto be understood that such modifications and variations are to beincluded within the purview of this application and the spirit and scopeof the appended claims.

We claim:
 1. A method for measuring formaldehyde emissions from acomposite wood product bonded with a urea-formaldehyde resin adhesive,said method comprising:flowing air for less than about 60 minutes overat least one board sample of said composite wood product containedwithin a sample chamber, said sample chamber having a volume of lessthan about 0.5 m³, to form a formaldehyde-containing air stream; and,thereafter passing a portion of said formaldehyde-containing air streamover an electrochemical formaldehyde sensor to measure the concentrationof formaldehyde in said air stream, wherein said electrochemical sensorcan determine the formaldehyde content in said formaldehyde-containingair stream with a sufficiently low level of formaldehyde removal fromsaid formaldehyde-containing air stream as to leave the formaldehydeconcentration essentially undisturbed in said formaldehyde-containingair stream.
 2. A method according to claim 1 further comprising the stepof adjusting the temperature to within the range of 75°-77° F. and therelative humidity to within 46-54% before passing air over said at leastone board sample.
 3. A method according to claim 1 wherein said air isflowed over said at least one board sample for a period of less than 15minutes before passing said portion over said sensor.
 4. A method formeasuring steady-state formaldehyde emissions from a composite woodproduct bonded with a urea-formaldehyde resin adhesive, said methodcomprising:recirculating air for less than about 30 minutes over atleast one board sample of said composite wood product contained within asample chamber to form a formaldehyde-containing recirculating airstream; introducing ambient make-up air into said recirculating airstream; and thereafter passing a portion of said formaldehyde-containingrecirculating air stream over an electrochemical formaldehyde sensor tomeasure the concentration of formaldehyde in said recirculating airstream, wherein said electrochemical sensor can determine theformaldehyde content in said formaldehyde-containing recirculating airstream with a sufficiently low level of formaldehyde removal from saidformaldehyde-containing recirculating air stream as to leave theformaldehyde concentration essentially undisturbed in saidformaldehyde-containing recirculating air stream.
 5. A method accordingto claim 1 further comprising the step of adjusting the temperature towithin the range of 75°-77° F. and the relative humidity to within46-54% before passing air over said at least one board sample.
 6. Amethod according to claim 4 wherein said air is flowed over said atleast one board sample for a period of less than 15 minutes beforepassing said portion over said sensor.
 7. A method according to claim 4wherein said ambient make-up air is filtered before introducing saidmake-up air into said recirculating air stream.
 8. A method according toclaim 4 wherein said air is recirculated over said at least one boardsample for a time sufficient to establish steady-state conditions insaid sample chamber.
 9. A method according to claim 4 wherein said airis recirculated over said at least one board sample for a timesufficient to reach about 80 to 90% of steady-state conditions in saidsample chamber.
 10. A method for measuring equilibrium formaldehydeemissions from a composite wood product bonded with a urea-formaldehyderesin adhesive, said method comprising:recirculating air over at leastone board sample of said composite wood product contained within aclosed sample chamber for a time sufficient to establish at least about80 to 90% of equilibrium emission conditions in said chamber and form anequilibrium formaldehyde-containing recirculating air stream; thereafterpassing a portion of said equilibrium formaldehyde-containingrecirculating air stream over an electrochemical formaldehyde sensor tomeasure the concentration of formaldehyde in said recirculating airstream; and returning said portion to said sample chamber after saidmeasurement, wherein said electrochemical sensor can determine theformaldehyde content in said formaldehyde-containing recirculating airstream with sufficiently low level of formaldehyde removal from saidformaldehyde-containing recirculating air stream as to leave theformaldehyde concentration essentially undisturbed in saidformaldehyde-containing recirculating air stream.
 11. A method accordingto claim 10 further comprising the step of adjusting the temperature towithin the range of 75°-77° F. and the relative humidity to within46-54% before passing air over said at least one board sample.